1. Field of the Invention
The present invention relates to a static capacitive microsensor especially enabling determination of a distance variation between the microsensor and a charged surface having a relative motion with respect to the microsensor.
The present invention especially applies to a fingerprint detector.
2. Discussion of the Related Art
FIG. 1 shows a conventional capacitive microsensor for detection of fingerprints. This microsensor includes a set of detection sites 1 arranged in one or several rows. The microsensor is formed on an insulating wafer 2, for example made of glass or silica or a flexible material. On wafer 2 are formed conductive regions 3, end areas 4 of which are placed in front of the detection sites. Conductive regions 3 are coated with an insulating layer 5 above which is formed a conductive layer 6 including windows in front of the detection sites. Conductive layer 4 is coated with an insulating layer 7 coated with a conductive layer 8 also including windows at the level of each detection site. The substrate is crossed by conductive vias. First vias 11 contact regions 3, a second via 12 contacts layer 6, and a third via 13 contacts layer 8. Each of the vias ends with a connection pad, respectively 14, 15, 16. It should be understood that there exists one detection area 4 for each window 1, while conductive layer 6 and conductive layer 8 are common to the entire structure. For practical reasons, the lower surface of substrate 2 is coated with an insulating layer 18, this insulating layer being possibly coated with a conductive shielding layer (not shown) open at the pad locations.
Regions 3 are connected to a detection circuit by pads 14. Layer 6 is connected to ground by pad 15. Layer 8 is connected to a high-frequency generator by pad 16. Thus, when an isolating surface, for example, a finger, is moved with respect to microsensor row 1, this surface is charged by the high-frequency signal and the capacitance between the surface portion passing above a detection area and this detection area varies according to the distance between the surface portion passing above the detection area (hollow or relief of a fingerprint) and this area. The successive relieves of a fingerprint may thus be detected at the location of each detection area. Various means are known in the art to perform the measurements. Some of these means are described in European patent application 1058531 of Idex AS Company, which is incorporated herein by reference.